9,10-Diarylanthracenes as stable electrochemiluminescent emitters in water

Article abstract of DOI:10.1021/jo301616t

Two hydrophilic diarylanthracenes, explicitly 9,10-bis(N-methylimidazolium- 3-propoxyphenyl)anthracene (DAA1) and 9,10-bis(N-methylimidazolium-3-propoxy-2, 6-dimethylphenyl)anthracene (DAA2), are synthesized and fully characterized. Both are found to be soluble in aqueous medium and to exhibit optical properties similar to those of the parent 9,10-diphenylanthracene, whose solubility is virtually negligible in water. The detailed analysis of their photochemical stability as well as electrochemical and electrochemiluminescent properties reveals that the sterically highly shielded anthracene DAA2 shows inertness toward reactions with singlet oxygen and OH^- ions during photo- and electrochemical initiation and stable ECL emission in aqueous medium.

Full text of DOI:10.1021/jo301616t

Article
pubs.acs.org/joc
9,10-Diarylanthracenes as Stable Electrochemiluminescent Emitters
in Water
Palani Natarajan* and Michael Schmittel*
Center of Micro- and Nanochemistry and Engineering, Organische Chemie I, Universitat Siegen, Adolf-Reichwein-Straße 2, D-57068
̈
Siegen, Germany
S
* Supporting Information
ABSTRACT: Two hydrophilic diarylanthracenes, explicitly 9,10-
bis(N-methylimidazolium-3-propoxyphenyl)anthracene (DAA1)
and 9,10-bis(N-methylimidazolium-3-propoxy-2,6-dimethyl-
phenyl)anthracene (DAA2), are synthesized and fully charac-
terized. Both are found to be soluble in aqueous medium and to
exhibit optical properties similar to those of the parent 9,10-
diphenylanthracene, whose solubility is virtually negligible in
water. The detailed analysis of their photochemical stability as
well as electrochemical and electrochemiluminescent properties
reveals that the sterically highly shielded anthracene DAA2 shows
inertness toward reactions with singlet oxygen and OH− ions
during photo- and electrochemical initiation and stable ECL
emission in aqueous medium.
phosphate buffer. Thus, the design and development of stable
ECL emissive diarylanthracenes exhibiting both water solubility
and high stability against nucleophilic attack on the radical cation
stage has been an ongoing challenge for many years.3−8
INTRODUCTION
■
Among the large number of organic emitters studied in
electrochemiluminescence (ECL), 9,10-diphenylanthracene
(DPA) and its derivatives exhibit remarkable luminescence
properties in organic media.1 In co-reactant ECL, DPA yields a
bright blue emission with an ECL efficiency of 25%, which
represents the theoretical maximum for ECL from DPA
singlets.1b As a result, DPA has been used as an ECL standard
in aprotic media.1,2 However, neither DPA nor its derivatives
have been used in biological fluids. This area is dominated so far
by well-known inorganic ECL emitters, such as the tris(2,2′-
bipyridyl)ruthenium(II), [Ru(bpy)3]2+.1b,c Typical diarylanthra-
cenes are characterized by a (i) lack of any solubility in aqueous
solutions, (ii) high reactivity of the radical cation state toward
water, and (iii) lack of photostability due to facile oxygenation at
the meso-positions.3 Therefore, several attempts have been made
to initiate ECL emission from DPA and its derivatives in aqueous
solutions.4 For example, Bard et al.5 have introduced hydrophilic
DPA derivatives, such as sodium 9,10-diphenylanthracene-2-
sulfonate (DPAS), that show a good solubility in aqueous
solutions. However, DPAS and its radical ion lack chemical
stability during electrolysis, thus yielding unstable ECL signals. In
addition, Bard et al.1j,6 produced dispersed nanoparticles (also
known as quantum dots) of DPA, but their ECL emission
intensity in aqueous media was rather weak. The failure was
assigned to the small diffusion coefficient of DPA-infused
nanoparticles in aqueous solutions. Recently, Chen et al.7
reported on DPA-doped polystyrene beads that were soluble in
water up to a concentration of 10−3 M while showing reasonable
ECL emission. Nonetheless, their ECL intensity rapidly
decreased due to nucleophilic OH−/water attack in aqueous
9,9′-Bianthryls3,9 and trianthryls exhibit a much better
photostability against singlet oxygen (1O2) than simple aryl- or
diarylanthracenes,8 a finding that suggests that steric bulk may
equally prevent anthracenes from undergoing photo-oxidation.
In our laboratories, we established that copper(I) ions are much
more inert toward oxidation when they are coordinated to bulky
2,9-bis(2,6-dimethylphenyl)-1,10-phenanthroline ligands,10 as
the ortho-methyl groups in the 2,6-dimethylaryl unit provide
good steric protection against oxygen/water attack. On the basis
of these observations and our experience in ECL,11 we herein
introduce the water-soluble and sterically hindered anthracene
DAA2, 9,10-bis(N-methylimidazolium-3-propoxy-2,6-
dimethylphenyl)anthracene (Chart 1), as a useful water-stable
ECL emitter. For comparison, DAA1 will be prepared and its
photochemical and electrochemical stability as well as its
luminescence activity in organic and aqueous solvents shall be
evaluated. Our design considerations were based on the
observation that the electron-deficient imidazolium groups
should not affect luminescence quantum yields and radical
cation stability as manifested from other anthracene derivatives
decorated by imidazolium groups.12
Received: August 3, 2012
Published: September 5, 2012
© 2012 American Chemical Society
8669
dx.doi.org/10.1021/jo301616t | J. Org. Chem. 2012, 77, 8669−8677

The Journal of Organic Chemistry
Article
Chart 1. Molecular Structure of Aqueous Soluble 9,10-Diarylanthracenes DAA1 and DAA2
a
Scheme 1. Synthetic Route for 9,10-Diarylanthracenes DAA1 and DAA2
a
The numbering in the structures is only used for the 1H NMR assignments, which are not necessarily in accordance with the IUPAC nomenclature
pattern.
Table 1. Optical Absorption, PL, PL Quantum Yields, Redox Potential, ECL, and Relative ECL Quantum Yield of DAA1 and DAA2
a
b
a
c
d
e
fg
,
λmaxabs (nm)
band gap λ (eV)
λmaxfl (nm)
Φfl
oxid. E°ox (V)
λmaxecl (nm)
Φrel,ECL
DAA1
DAA2
372sm
374sw
366sm
367sw
412 (3.01)sm
417 (2.97)sw
407 (3.05)sm
409 (3.05)sw
427sm
433sw
421sm
424sw
0.78sm
0.77sw
0.61sm
0.60sw
0.78, 1.02sd
431sd
435sw
423sd
425sw
7.72 0.3sd
7.73 0.7sw
6.84 0.1sd
6.83 0.1sw
0.94sw
0.71, 0.99sd
0.86, 1.07sw
a
b
Absorption and emission spectra are recorded for dilute solutions (10 μM). Optical band gap values are calculated from the tail end absorption.
c
d
Quantum yields are calculated using 9,10-diphenylanthracene as a standard with λexc = 365 nm. All potentials are obtained from cyclic voltammetry
e
and referenced versus Fc/Fc+ at a scan rate of 100 mV s−1. Electrochemiluminescence is measured using tris-n-propylamine as a co-reactant (50
f
g
mM). Φrel,ECL is the relative ECL compared to [Ru(bpy)3]2+, which is taken as unity. Values are the average of at least 5−6 independent
experiments: sm, experiment performed in methanol; sw, experiment performed in aqueous buffer; sd, experiment performed in DMF.
mediated Suzuki coupling reaction between 9,10-dibromoan-
thracene and 4-methoxyphenylboronic acid afforded 9,10-bis(4-
methoxyphenyl)anthracene, which was demethylated with BBr3,
furnishing the 9,10-bis(4-hydroxyphenyl)anthracene. Ether-
RESULTS
■
Synthesis and Characterization. The 9,10-diarylanthra-
cenes DAA1 and DAA2 (Chart 1) were readily synthesized
according to the routes presented in Scheme 1. A Pd(PPh3)4-
8670
dx.doi.org/10.1021/jo301616t | J. Org. Chem. 2012, 77, 8669−8677

The Journal of Organic Chemistry
Article
Figure 1. Absorption (left) and fluorescence (right, λexc = 365 nm) spectra of DAA1 and DAA2 in methanol and aqueous buffer solutions. Inset on the
left shows the expanded visible portion of DAA1 and DAA2. Color codes: In aqueous solution: DAA1, black; DAA2, green. In methanol; DAA1, red;
DAA2, blue.
Figure 2. Cyclic voltammograms of 0.5 mM DAA1 (left) and DAA2 (right) in aqueous (blue) and DMF (red) solutions at 100 mV s−1 scan rate.
ification of the latter bisphenol with excess of 1,3-dibromopro-
pane was followed by reaction with N-methylimidazole to deliver
DAA1 in 64−66% yield (Scheme 1). A similar protocol starting
from 9,10-bis(4-methoxy-2,6-dimethylphenyl)anthracene led to
DAA2 (Scheme 1); the 9,10-bis(4-methoxy-2,6-
dimethylphenyl)anthracene precursor was, in turn, prepared by
a NiCl2(PPh3)2-mediated coupling reaction between 9,10-
dibromoanthracene and 4-methoxy-2,6-dimethylphenylmagne-
sium bromide.13 All of the compounds were purified by column
chromatography. All intermediate compounds and DAA1 and
DAA2 were characterized by the usual spectroscopy techniques
(Supporting Information).
DAA1 and DAA2 are both readily soluble in aqueous (2−2.5
mM) as well as in alcoholic solutions, whereas they lack solubility
in common organic solvents, such as hexane, dichloromethane,
chloroform, ethyl acetate, toluene, etc. Therefore, the following
experiments were performed either in dimethylformamide
(DMF), methanol, or aqueous phosphate buffer solutions (pH
= 7.0 at 20 °C) as needed.
by analogy.3,14 The two absorption bands in the 210−270 nm
region are attributed to the substituted aryl groups. The optical
band gap energy, as calculated from the tail end of the absorption,
varies from 2.97 to 3.05 eV (Table 1). A PL study of DAA1 and
DAA2 shows a strong emission in the 390−520 nm range (blue
emission, Figure 1) that is independent of the excitation
wavelength. Notably, no aggregation induced red shift emission
is observed neither from DAA1 nor from DAA2, even at high
concentration (1 × 10−4 M, cf. Supporting Information).1l As
expected, DAA2 exhibits blue-shifted absorption and emission
maxima due to the reduced conjugation as compared to DAA1.
PL quantum yields (Φfl) are determined using 9,10-diphenylan-
thracene (DPA, Φ = 0.9 in cyclohexane) as a standard.15 They
amount to 0.77−0.78 and 0.60−0.61 for DAA1 and DAA2,
respectively (Table 1).
Photostability. In order to establish the photostability of
DAA1 and DAA2, photo-oxygenation reactions are conducted
separately in oxygen-saturated aqueous and methanol solutions
in the presence of catalytic methylene blue as photosensitizer.
After 4−5 h, DAA1 affords the corresponding 9,10-endoperoxide
as a main product, which is isolated and characterized by NMR
analysis. The photo-oxidation is readily monitored by absorption
and PL because their peak intensities decreased gradually with
time as a result of the breakdown of the anthracene core
(Supporting Information). In contrast, DAA2 is found to be inert
Photophysical Data. The UV−vis absorption and photo-
luminescence (PL) of DAA1 and DAA2 are studied in methanol
and aqueous buffer solutions (for a summary of the results, see
Table 1). The UV−vis spectra of DAA1 and DAA2 (Figure 1)
exhibit similar absorption peaks between 315 and 420 nm that
are assigned to π−π* transitions of the central anthracene core
8671
dx.doi.org/10.1021/jo301616t | J. Org. Chem. 2012, 77, 8669−8677

The Journal of Organic Chemistry
Article
Figure 3. DFT (6-311G*) optimized structure of DAA1+• (the imidazolium substituents are omitted in the above drawing). Notice that the C9 and C10
atoms (green) are readily accessible without steric crowding.
Figure 4. DFT (6-311G*) optimized structure of DAA2+• (the imidazolium substituents are omitted in the above drawing). Notice that the C9 and C10
atoms (green) are shielded by the ortho-methyl groups of the phenyl substituents.
under photo-oxidation conditions, even after 20 h of irradiation,
as confirmed by NMR and PL analysis.
radical cation (DAA1+• and DAA2+•) affects, in particular, the
orientation of aryl units in DAA1, as its dihedral angle changes
from 67 to 53°. In contrast, the dihedral angle remains unaltered
for DAA2 (89°) and its radical cation DAA2+• (88°) (Figure 4).
The calculated value of the dihedral angle for DAA1+• is rather
similar to the value of 57° reported for DPA+• (produced by
dissolving DPA in hexane and treated with concentrated sulfuric
acid), determined by electron spin resonance spectroscopy
(ESR) and molecular model estimation.18 This suggests that the
level of theory used here is suitable for the system studied.
Electrochemiluminescence. ECL emission of DAA1 and
DAA2 in presence of tris-n-propylamine (TPrA, 50 mM) as co-
reactant is measured by reversibly scanning across the first
oxidation potential (0.15−0.95 V with respect to a silver wire as a
quasi-reference electrode, Table 1) in aqueous buffer as well as in
non-aqueous DMF solution.19 As shown in Figure 5, both DAA1
and DAA2 provide noticeable ECL emission even at low
concentrations (1−5 × 10−4 M), while no emission is observed in
the absence of either TPrA or luminophore. In line with their
different PL quantum yields, the ECL emission intensity of
DAA1 is 20% more than that observed from DAA2. The
emission maxima in ECL and PL of DAA1 and DAA2 are found
to be similar, indicating that the emission occurs from the same
excited states (Table 1). In aqueous buffer solution, the ECL
signal is generated by reversibly sweeping the electrode potential
in the range of 0.15 to 0.95 V in the presence of 50 mM TPrA as a
co-reactant at a scan rate of 100 mV s−1. However, ECL emission
intensity diminishes with each subsequent scan due to electrode
passivation as a consequence of irreversible oxidation of co-
reactant and luminophore. Therefore, for each resultant
experiment conducted in aqueous buffer solution, the electrode
surface was mechanically polished using an alumina slurry. After
polishing, DAA2 shows an ECL signal without any significant
drop of the emission intensity, at least for 100 repeated potentials
scans (Figure 6). On the other hand, the ECL emission intensity
Electrochemical Data. Oxidation potentials of DAA1 and
DAA2 are determined by cyclic voltammetry (CV) in DMF and
in aqueous buffer solution (Figure 2), and the results are
summarized in Table 1. Reduction potentials could unfortu-
nately not be determined due to structureless waves at negative
potential. DAA1 in aqueous buffer solution shows a quasi-
reversible oxidation wave at 0.94 V, whereas DAA2 exhibits two
reversible oxidation waves, the first at 0.86 V and the second at
1.07 V (scan rate = 100 mV s−1). Clearly, the cation radical of
DAA1 is unstable, as evidenced by the lack of chemical
reversibility. A similar behavior is observed at a scan rate of
1000 mV s−1 (Supporting Information). It is possibly due to the
reaction of the radical cation of DAA1 with water, vide infra. In
contrast, experiments with DAA1 and DAA2 in non-aqueous
DMF show two oxidation waves. The first wave is found to be
reversible for both DAA1 and DAA2, whereas the second
oxidation wave is reversible for DAA2 only (Figure 2). The
irreversible second oxidation of DAA1 in DMF indicates that the
dication (DAA12+) is unstable under the conditions used in this
study.
DFT Computations. DFT computational studies are
performed using the 6-311G* basis set in order to establish the
dihedral angles and the influence of steric hindrance on
photochemical and electrochemical properties of neutral and
radical cations of DAA1 and DAA2 (atomic coordinates and
absolute energies of all structures presented in the Supporting
Information, cf. Tables S1−S4).16 The calculated dihedral angles
between the aryl group and the central anthracene unit are 67 and
89° for DAA1 and DAA2, respectively (Supporting Informa-
tion). They are in good agreement with the value reported in the
literature for other diarylanthracenes as received from single-
crystal X-ray structures.17 As shown in Figures 3 and 4, the one-
electron oxidation of DAA1 and DAA2 to their corresponding
8672
dx.doi.org/10.1021/jo301616t | J. Org. Chem. 2012, 77, 8669−8677

The Journal of Organic Chemistry
Article
formation of byproducts, such as 9,10-bis(N-methylimidazo-
lium-3-propoxyphenyl)-9,10-dihydroanthracenediol, is con-
firmed by NMR20 and PL analysis. PL is monitored after each
scan of DAA1 in aqueous solution (after each scan 20 μL of
sample is removed and diluted to 2.5 mL for PL measurement).
The initial PL emission intensity between 350 and 500 nm drops
gradually due to destruction of the anthracene core.
Both DAA1 and DAA2 first undergo a reversible one-electron
oxidation in non-aqueous DMF as apparent from CV analysis.
Indeed, ECL behavior in DMF is markedly different from that
observed in aqueous solution of DAA1 and DAA2, vide infra.
The slightly blue-shifted ECL emission of DAA1 and DAA2 in
DMF as compared to that in aqueous solution (Figure 5 and
Table 1) is a result of a solvent effect that is equally manifested in
PL. Although the radical cations of DAA1 and DAA2 in DMF
show good stability during a single CV trace, the ECL intensity of
DAA1 decreases gradually with repeated scans. However,
intensity loss for every subsequent scan is much smaller (3−
5%) than that for DAA1 in aqueous solution (15−25%). ECL
emission of DAA1 in DMF reaches a somewhat constant value
after several repeated scans, which is maintained only for 3−4
further repeated pulses (Supporting Information). Afterward, the
intensity drops again for the subsequent scan. A similar behavior
has been observed with other diarylanthracene derivatives as
reported in the literature.1,2 In contrast, the ECL emission
intensity of DAA2 in DMF is found to be invariable for each
repeated scan, which is similar to the behavior observed in
aqueous solution. In other words, the ECL intensity of DAA2 is
very stable and independent of the medium as well as of repeated
oxidative scans (Figure 6). For the quantification of the ECL
Figure 5. Normalized ECL spectra of 0.5 mM DAA1 and DAA2 in
aqueous phosphate buffer (pH = 7 at 20 °C) as well as in DMF solutions.
ECL spectra were recorded in the presence of 50 mM TPrA as co-
reactant.
emission of DAA1 and DAA2, relative ECL efficiency (Φrel,ECL
,
integrated light intensity under an emission curve) was measured
using [Ru(bpy)3]2+ as a standard; the results are summarized in
Table 1. The values of Φrel,ECL of DAA1 and DAA2 are found to
be ca. 7-fold higher than the standard Ru complex.
DISCUSSION
Figure 6. ECL emission spectrum of DAA2 in aqueous phosphate buffer
(pH = 7 at 20 °C) solution taken with repeated scans (100 cycles).
Notice that there is no change in ECL intensity.
■
The high luminescence quantum yield and facile oxidation and
reduction behavior of diarylanthracenes applies to applications in
diverse areas including optoelectronic materials, sensors, electro-
luminescence, solar cells, etc.1,21 The UV−vis absorption and PL
spectra of DAA1 and DAA2 are measured in aqueous and
methanol solutions; they both resemble closely that of the parent
DPA.3,14 The λmax absorption and emission values are slightly
increased with solvent polarity in regard to the solvent effect on
photophysical properties of diarylanthracenes.1 A blue-shifted
of DAA1 is still diminished by about 15−25% for each following
scan in aqueous buffer solution (Figure 7), most likely due to the
instability of the oxidized DAA1 against water/oxygen. The
abs
absorption and emission maximum is observed for DAA2 (λmax
366 nm, λmaxem 422 nm) as compared to those of DAA1 (λmax
abs
373 nm, λmaxem 430 nm) as a result of the reduced conjugation
between the dimethylphenyl substituents and the anthracene
core in the former system. The dimethylphenyl groups in DAA2
twist out of the plane of the anthracene core by 89°, which is 22°
higher than the torsional angle between phenyl and anthracene in
DAA1 (67°), owing to higher steric interaction between the
−CH3 with the nearest hydrogens of the anthracene ring in
DAA2. Hence, the dimethylphenyl groups have weak
communication with the anthracene when compared to DAA1.
The lack of planarity between phenyl rings and anthracene
inhibits excimer formation of DAA1 and DAA2 in solution
(Supporting Information). The fluorescence quantum yields of
DAA1 and DAA2 vary from 0.60 to 0.78 in aqueous and organic
solutions. One may notice that the imidazolium residues in
DAA1 and DAA2 do not affect the electronic and optical
Figure 7. ECL emission spectrum of DAA1 in aqueous phosphate buffer
(pH = 7 at 20 °C) solution taken with repeated potential scans. Notice
that the ECL intensity decreases with every subsequent scan.
8673
dx.doi.org/10.1021/jo301616t | J. Org. Chem. 2012, 77, 8669−8677

The Journal of Organic Chemistry
Article
properties of the diarylanthracene core,12 which is apparent from
similar spectral features with those of the parent DPA (Figure 1).
The marginal differences in PL quantum yields of DAA1 and
DAA2 as compared to DPA may be due to the greater internal
conversion of the n-propyl substituents in the former
chromophores. Thus, DAA1 and DAA2 conserve the electronic
and photophysical properties of DPA.
anthracene.23 Partial reversibility of the second electron
oxidation as exhibited by DAA2 in aqueous and dry DMF may
be due to a lack of additional substitutions at C2 and C6 positions
of the anthracene core. Electrochemical investigations of
anthracenes functionalized at C2, C6, C9, and C10 positions
show complete reversibility for both first and second electron
transfer.22,24 We expect that the instability and partial stability of
DAA12+ and DAA22+, respectively, will not affect the quality of
ECL since ECL is generated by spanning the electrode potential
slightly beyond the first oxidation peak potential.1 In particular,
co-reactant-mediated ECL, if properly using the correct
switching potential, is not usually affected by the second redox
potential.1b,19
Due to the application of DPA as a precursor in different fields,
the structures of neutral, cation, and anion radicals of DPA have
previously been studied in great detail, utilizing different types of
experimental and theoretical techniques.17,18,22 The DFT-
optimized geometries of DAA1 and DAA2 and their radical
cations closely resemble previously published results. As depicted
in Figure 4, the ortho-CH3 groups of the phenyl unit in DAA2,
due to severe steric interactions with protons 1H, 4H, 5H, and
8H of the anthracene, assume an orientation, in which the top
and bottom of the reactive meso-carbons (C9 and C10) are well
shielded against attack (e.g., 1O2). Similar properties have been
reported earlier for other sterically congested diarylanthra-
cenes.8,24
One of the frequent drawbacks of diarylanthracenes is their
lack of photostability due to facile oxygenation at the meso-
positions of the anthracene core.3 Although our interest was to
explore the advantages of the sterically congested DAA2 over
DAA1 as an ECL probe, the study of their photostability (DAA1
and DAA2) allowed us to judge on the impact of the CH3 groups
at the 2,6-positions of the phenyl unit more clearly. Judging from
the photo-oxygenation study, DAA2 is much superior to DAA1
with respect to photostability. It is understandable that steric
repulsion between oxygen and the proximal ortho-CH3 groups at
the phenyl rings of DAA2 disfavors 9,10-endoperoxide
formation. A similar trend is reported for the anti-isomer of
9,10-bis(o-tolyl)anthracene, whose photostability was reported
to be higher than that of DPA.8c DAA1 not only lacks
photostability but also undergoes ready oxygenation at the
meso-positions, similar to DPA (Supporting Information). It is
worth noting that the predominant formation of 9,10-
endoperoxide in the case of DAA1 implies that the C9 and
C10 atoms of the anthracene core constitute the most electron-
rich site, similar to the situation with DPA. In contrast,
insensitivity of DAA2 against oxygen is clearly a result of steric
hindrance and not related to electronic effects.8c
The effect of steric hindrance induced by the 2,6-
dimethylphenyl groups of DAA2 on the reactive anthracene
radical cations is illustrated by CV experiments. During the
electrolysis, DAA1 exhibits irreversible electron transfer in
aqueous solution due to nucleophilic attack by water, resulting in
the formation of byproduct 9,10-bis(N-methylimidazolium-3-
propoxyphenyl)-9,10-dihydroanthracenediol as established by
NMR analysis. In contrast, DAA2 shows two consecutive one-
electron oxidation waves in a reversible fashion to DAA2+•and
partial reversibility to DAA22+, in aqueous solution (Figure 2).
Normally, aromatic radical cations are extremely reactive species;
therefore, only in few cases reversible redox behavior of either
cation or dication has been reported in aqueous or acidic
medium.1,18,22 Our results clearly demonstrate that one may
generate stable aryl cations in aqueous medium by installing
appropriate shielding groups at the reactive positions of the
chromophore/electrophore.
In order to understand the electrochemical behavior of DAA1
and DAA2 in non-aqueous medium, electrolysis was conducted
in DMF solution. Reversible one-electron oxidation generates
radical cations (DAA1+• and DAA2+•), while further oxidation to
the dication is successful exclusively for DAA2. In case of DAA1,
an irreversible second oxidation wave is observed. The
irreversible redox behavior of DAA1 in non-aqueous DMF
clearly indicates that the dication (DAA12+) is unstable on the
time scale of CV. Therefore, it is likely that the dimethylphenyl
substitution in DAA2 not only is responsible for the suppression
of nucleophilic attack but also blocks any annihilation process by
stabilizing the radical ions via nonbonded interactions. This is
further evidenced by the analogous substitution effect reported
for several anthracene derivatives.8,23 For example, cations of
9,10-diarylanthracenes were reported to be more stable than
those of 9-arylanthracenes and anthracene. Likewise, 9-
arylanthracenes form more stable cations than the unsubstituted
The optimized geometry of DAA1+• and DAA2+• shows a
change in the orientation of the phenyl groups for DAA1+• only.
The torsional angle changes from 67 to 53° upon oxidation of
DAA1, due to the need of charge delocalization in DAA1+•,18b as
apparent from ESR analysis in related cases.8,18a In contrast, the
dihedral angle remains unaltered for DAA2 (89°), and its radical
cation DAA2+• (88°) as an angle change would cause severe
intramolecular steric hindrance. As in the neutral molecule,
DAA2+• is well shielded against attack.24
Good ECL behavior of DAA1 and DAA2 is realized in
aqueous and DMF solution containing TPrA as a co-reactant.19
At a scan rate of 100 mV s−1 (potential range = 0.15 to 0.95 V),
DAA1 and DAA2 in DMF provide ECL signals that are similar to
that for DPA, indicating that the imidazolium substituents do not
affect the electronic situation.12 The comparable emission
energies detected in the PL and ECL spectra indicate that the
same excited state is formed in both experiments. In the case of
DAA1, ECL is not stable for subsequent scans in aqueous and
DMF solution. Even when the electrode is repolished for each
scan, ECL emission is not restored, suggesting that the decay is
caused by formation of byproducts, such as 9,10-bis(N-
methylimidazolium-3-propoxyphenyl)-9,10-dihydroanthracene-
diol. Furthermore, the decomposition of DAA1 with each scan in
ECL may be monitored by PL, as the initial fluorescence intensity
of DAA1 decreases versus the number of repeated scans. Even in
DMF, DAA1+• undergoes decomposition to unstable products
as revealed by NMR (1H NMR shows highly complicated
aromatic region). So far, the exact reason for the loss of ECL
efficiency in non-aqueous DMF is unknown, but a similar
behavior was observed previously with other diarylanthracene
derivatives in non-aqueous CH3CN, C2H4Cl2, fluorobenzene,
etc.1,2,25 On the other hand, the sterically hindered anthracene
DAA2 produces a stable ECL emission for more than 100
repetitive scans, independent of the medium. Thus, ECL,
photochemical, and electrochemical results of DAA2 in aqueous
and non-aqueous medium clearly indicate that steric hindrance
caused by the 2,6-dimethylphenyl groups actively protects the
8674
dx.doi.org/10.1021/jo301616t | J. Org. Chem. 2012, 77, 8669−8677

The Journal of Organic Chemistry
Article
under the emission curves) of the sample and the standard, and ηu and ηs
to the refractive indexes of the corresponding solutions (pure solvents
are assumed; water 1.333, methanol 1.329, and cyclohexane 1.427).
Cyclic voltammograms were obtained using a standard three-
electrode setup, that is, a 1 mm platinum disk working electrode, a
platinum wire counter electrode, and a silver wire as pseudoreference
electrode. The working electrode in each case was polished on a felt pad
with alumina slurry and then rinsed with water followed by acetone and
subsequently dried with air. The potentials were measured at a scan rate
of 100−200 mV s−1. All solutions for the electrochemical experiments
contained the compounds of interest, DAA1 or DAA2 at 5 × 10−4 M and
tetra-n-butylammonium hexafluorophosphate (TBAP) as the support-
ing electrolyte (0.1 M). Ferrocene (Fc) was used as the internal standard
in non-aqueous DMF solution.
Electrochemiluminescence measurements were performed using a 5
× 10−4 M solution of DAA1 and DAA2 in DMF or aqueous buffer
solution containing additionally tris-n-propylamine (TPrA, 50 × 10−3
M) as a co-reactant. A similar electrode setup as in the CV experiment
was used. To generate ECL, the potential was swept across the first
oxidation potential (0.15−0.95−0.15 V with respect to a silver wire as a
quasi-reference electrode) at 100 mV s−1. The resulting emission spectra
were recorded with a CCD camera cooled at −130 °C. The relative ECL
efficiency of DAA1 and DAA2 was determined by using the equation30
reactive C9 and C10 positions of the anthracene core against
nucleophilic attack and intermolecular annihilation. Similar
features have been reported for two other sterically congested
ECL emitters, namely, meso-tetrakis(3-sulfonatomesityl)-
porphyrin26 and 2,6-bis(9-[1,1′-biphenyl]-9-[p-tolyl]fluoren-2-
yl)-9,10-di[p-tolyl]anthracene24 in aqueous and organic solution,
respectively. The values of relative ECL intensities Φrel,ECL for
DAA1 and DAA2 in aqueous and DMF solutions are given in
Table 1. It should be noted that both DAA1 and DAA2 exhibit
higher ECL efficiency than the standard [Ru(bpy)3]2+, in
agreement with their higher PL quantum yields.24,27 Comparing
DAA1 and DAA2, the former shows a 10% higher efficiency than
latter due to the absence of −CH3 substitutions that may cause
energy dissipation by rotation.
As mentioned at the onset, only [Ru(bpy)3]2+-based ECL
emitters have commercially been used in bioanalysis.1b,c Usual
drawbacks for other reported ECL luminophores are limited
water solubility and lack of photo- as well as electrochemical
stability.1b As an alternative, hydrophobic ECL luminophores
were encapsulated in either nanoparticles or polymer blends to
enable ECL in aqueous media.4,6,28 Using appropriately shielded
molecular luminophores that are inert under aqueous conditions
seems to be a much more straightforward strategy.26 Water-
soluble organic fluorophores, including DAA1 and DAA2 as
reported here, exhibit higher ECL efficiency and readily
overcome the disadvantages existing with the more familiar
ruthenium (inorganic)-based ECL emitters, such as environment
unfriendliness, limited availability, etc.29
In summary, the water-soluble diarylanthracenes DAA1 and
DAA2 are successfully prepared by introducing positively charge
imidazolium groups at remote positions of the luminophore.
Both DAA1 and DAA2 exhibit optical and electrochemical
properties similar to that of the parent blue emitter 9,10-
diphenylanthracene. The dimethylphenyl substitution in DAA2
imparts substantial photochemical and electrochemical stability
to the anthracene unit. As a result, stable ECL emission of DAA2
is observed even in aqueous medium during co-reactant-
mediated oxidation. These findings illustrate that the use of
steric hindrance as a tool to protect active sites of a luminophore
against undesired electrochemical reactions is a successful
addition to ECL design strategies.
Φ ,
= Φ°ECL(I/Q)(Q°/I°)
rel ECL
where Φrel,ECL and Φ°ECL are the ECL efficiency of the target and
standard samples, I and I° are the integrated ECL intensities (area under
a curve) of the target and standard systems, and Q and Q° are the charges
consumed by the target and standard, respectively. We used
[Ru(bpy)3]2+ as a standard with the Φrel,ECL value of 1.30 In order to
maintain an equal charge transfer for the target emitter co-reactant and
standard co-reactant systems during ECL,30 we performed experiments
at exactly similar conditions (i.e., same electrode, solvent, electrolyte, co-
reactant, and concentrations).
Preparation of 9,10-Bis(4-methoxyphenyl)anthracene. 9,10-
Dibromoanthracene (1.0 g, 3.0 mmol), 4-methoxyphenylboronic acid
(1.0 g, 6.6 mmol), and [Pd(PPh3)]4 (0.21 mg, 6.0 mol %) were
introduced into an initially oven-dried pressure tube under N2. To this
mixture were added toluene (20 mL) and saturated NaHCO3 solution
(7 mL). The reaction mixture was refluxed at 90−100 °C for 12 h. Then
the contents were extracted with CH2Cl2, dried over Na2SO4, and the
solvent was evaporated. The resultant material was purified by silica gel
column chromatography using 5−10% CH2Cl2 in hexane, yielding 9,10-
bis(4-methoxyphenyl)anthracene as a pale yellow solid (0.92 g, 82%):
mp 272−274 °C (mp 274 °C);31 Rf 0.22 in hexane; 1H NMR (400 MHz,
CDCl3) δ 3.97 (s, 6H, 5-H), 7.15 (d, J = 8.8 Hz, 4H, 3-H), 7.32−7.34
(m, 4H, 2-H), 7.40 (d, J = 8.8 Hz, 4H, 4-H), 7.73−7.75 (m, 4H, 1-H);
13C NMR (100 MHz, CDCl3) δ 55.1, 113.5, 124.6, 126.7, 129.9, 130.8,
EXPERIMENTAL SECTION
■
General Aspects. All reactions were performed under a nitrogen gas
atmosphere in oven-dried glassware. Dry tetrahydrofuran, benzene, and
toluene were freshly distilled over sodium prior to use. All reactions were
monitored by analytical thin layer chromatography (TLC) on silica gel.
Column chromatography was conducted with silica gel (60−120 mesh).
132.1, 136.4, 158.7.
Preparation of 9,10-Bis(4-methoxy-2,6-dimethylphenyl)-
anthracene. This compound was prepared according to a previously
reported protocol.13 To a solution of 9,10-dibromoanthracene (1.0 g,
3.0 mmol) and dry NiCl2(PPh3)2 (0.30 mg, 10.0 mol %) in 50 mL of
THF at −35.0 °C was added 4-methoxy-2,6-dimethylphenylmagnesium
bromide (2.4 equiv) in THF over 1.5 h. The resultant dark green
solution was brought to room temperature and refluxed at 80 °C for 36
h. Subsequently, the reaction mixture was quenched with 10% aqueous
HCl and extracted with ethyl acetate, dried over Na2SO4, and the
solvents were evaporated. The brown residue was purified by column
chromatography over silica gel using 5−10% CH2Cl2 in hexane to
furnish 9,10-bis(4-methoxy-2,6-dimethylphenyl)anthracene as a white
powder (0.42 mg, 32%): mp 247−251 °C; Rf 0.18 in hexane; 1H NMR
(400 MHz, CDCl3) δ 2.42 (s, 12H, 4-H), 3.91 (s, 6H, 5-H), 7.11 (s, 4H,
3-H), 7.33−7.35 (m, 4H, 2-H), 7.75−7.77 (m, 4H, 1-H); 13C NMR
(100 MHz, CDCl3) δ 16.2, 59.9, 124.8, 127.1, 130.0, 130.6, 131.7, 134.4,
136.9, 156.3. Anal. Calcd for C32H30O2: C, 86.06; H, 6.77. Found: C,
86.08; H, 6.53.
All commercial chemicals were used as received. All measurements were
carried out at ambient conditions unless stated otherwise. H and 13
C
1
NMR spectra were recorded on a 400 MHz spectrometer in deuterated
solvents.
UV−vis absorption spectra were recorded using a 10 μM solution of
DAA1 or DAA2 in either methanol or aqueous buffer solution. PL
spectra and quantum yields (ΦPL) were measured with excitation and
emission slits of either 1.5 or 2.5 nm. The concentration used was 10
μM, and ΦPL values were determined using 9,10-diphenylanthracene as
a standard (Φ = 0.9 in cyclohexane, λexc = 365 nm).15 The following
equation was used for the calculation of ΦPL for DAA1 and DAA2:
ϕ = ϕ(Iu/Is)(As/Au)(η /η )2
u
s
u
s
where the subscripts “s” and “u” refer to standard and unknown samples,
Au and As to absorbances of the sample and the standard at the excitation
wavelength, Iu and Is to the integrated emission intensities (i.e., areas
Preparation of 9,10-Bis(4-hydroxyphenyl)anthracene. To a
solution of 9,10-bis(4-methoxyphenyl)anthracene (0.90 g, 2.3 mmol) in
15.0 mL of CH2Cl2 at 0 °C was added dropwise 1.0 M BBr3 solution in
8675
dx.doi.org/10.1021/jo301616t | J. Org. Chem. 2012, 77, 8669−8677

The Journal of Organic Chemistry
Article
CH2Cl2 (5.1 mL) under N2 gas atmosphere. The reaction mixture was
allowed to stir overnight. Subsequently, it was quenched with 10%
aqueous HCl, extracted with ethyl acetate, dried over Na2SO4, treated
with charcoal, filtered, and concentrated. The pure product was obtained
as a colorless solid after filtration over a short pad of silica gel using 50%
ethyl acetate in hexane, followed by recrystallization from ethyl acetate
and hexane furnishing 9,10-bis(4-hydroxyphenyl)anthracene as a
greenish-yellow solid (0.71 g, 87%): mp 278−282 °C dec.; Rf 0.31 in
Preparation of DAA2. This compound was prepared starting from
9,10-bis(2,6-methyl-4-bromopropoxyphenyl)anthracene by following
the procedure described above (yield 71%): mp 286−290 °C dec.; Rf
0.12 in methanol; IR (KBr cm−1) 3170, 3125, 2934, 2862, 1575.6, 1469,
1382, 1169, 1113, 839, 740, 651, 624, 558; 1H NMR (400 MHz,
CD3OD) δ 2.40 (s, 12H, 4-H), 2.53 (quint, J = 6.4 Hz, 4H, 6-H), 4.00 (s,
6H, 8-H), 4.09 (t, J = 6.0 Hz, 4H, 5-H), 4.64 (t, J = 7.2 Hz, 4H, 7-H), 7.08
(s, 4H, 3-H), 7.30−7.32 (m, 4H, 2-H), 7.62−7.67 (m, 6H, 1\9-H), 7.83
(t, J = 2 Hz, 2H, 10-H), 9.13 (s, 2H, 11-H); 13C NMR (100 MHz,
CD3OD) δ 16.7, 32.1, 36.6, 48.6, 69.9, 123.9, 125.1, 126.0, 127.9, 131.3,
132.2, 132.9, 136.2, 138.0, 156.5; ESI-MS m/z (%) 809.3 (100) [M −
PF6]+. Anal. Calcd for C44H48F12N4O2P2: C, 55.35; H, 5.07; N, 5.87.
Found: C, 55.28; H, 5.01; N, 5.87.
Photo-oxidation of Diarylanthracenes DAA1 and DAA2. To a
solution of DAA1 or DAA2 in either water or methanol was added
methylene blue (catalytic amount: <1.0 mg) followed by bubbling
oxygen through the solution over 15−20 min. The resultant clear
solution was irradiated with a xenon lamp at 20 °C. Progress of the
reaction was monitored for up to 16−20 h by fluorescence decay. After
completion of the reaction, the solvent was removed under vacuum at
room temperature and the crude product was purified by recrystalliza-
tion in a water/acetone mixture. Molecule DAA2 did not yield any
endoperoxide, as concluded from NMR and fluorescence monitoring.
9,10-Endoperoxide from DAA1: Yield 96%; Rf 0.15 in methanol;
IR (KBr cm−1) 3059, 2920, 1491, 1458, 1437, 1383, 1246, 1024, 942,
767, 751, 666; 1H NMR (400 MHz, CD3OD) δ 2.39 (quint, J = 6.0 Hz,
4H, 6-H), 3.91 (s, 6H, 8-H), 4.10 (t, J = 5.6 Hz, 4H, 5-H), 4.45 (t, J = 6.8
Hz, 4H, 7-H), 6.96 (d, J = 8.4 Hz, 4H, 3-H), 7.12 (d, J = 8.8 Hz, 4H, 2-
H), 7.49−7.59 (m, 10H, 1\4\9-H), 7.68 (d, J = 2.0 Hz, 2H, 10-H), 9.02
(s, 2H, 11-H).
1
ethyl acetate−hexane, 4:1 ratio; IR (KBr, cm−1) 3366 (b, −OH); H
NMR (400 MHz, CD3OD) δ 6.98 (d, J = 12.4 Hz, 4H, 3-H), 7.14 (d, J =
12.4 Hz, 4H, 4-H), 7.21−7.23 (m, 4H, 2-H), 7.61−7.63 (m, 4H, 1-H);
13C NMR (100 MHz, CD3OD) δ 116.3, 125.8, 128.0, 131.2, 131.6,
133.4, 138.2, 158.1. Anal. Calcd for C26H18O2: C, 86.16; H, 5.01. Found:
C, 86.07; H, 5.12.
Preparation of 9,10-Bis(4-hydroxy-2,6-dimethylphenyl)-
anthracene. This compound was prepared from 9,10-bis(4-methoxy-
2,6-dimethylphenyl)anthracene following the same procedure as
described above (yield 89%): mp 271−273 °C; Rf 0.30 in ethyl
acetate−hexane (4:1); IR (KBr, cm−1) 3426 (b, −OH); 1H NMR (400
MHz, CD3OD) δ 2.34 (s, 12H, 4-H), 6.96 (s, 4H, 3-H), 7.26−7.29 (m,
4H, 2-H), 7.68−7.71 (m, 4H, 1-H); 13C NMR (100 MHz, CD3OD) δ
16.9, 125.6, 125.8, 128.2, 131.5, 131.6, 132.3, 138.4, 153.9. Anal. Calcd
for C30H26O2: C, 86.09; H, 6.26. Found: C, 86.06; H, 6.15.
Preparation of 9,10-Bis(4-bromopropoxyphenyl)-
anthracene. 9,10-Bis(4-hydroxyphenyl)anthracene (0.70 g, 1.9
mmol) was dissolved in 30 mL of acetone. After adding potassium
carbonate (1.6 g, 11.6 mmol) and 1,3-dibromopropane (0.6 mL, 5.8
mmol), the resultant heterogeneous solution was stirred for 8 h at room
temperature. Afterward, acetone was removed and the solid was
extracted with CH2Cl2. After drying over Na2SO4 and evaporation of the
solvents, the crude product was purified by silica gel column
chromatography using CH2 Cl2 yielding 9,10-bis(4-
bromopropoxyphenyl)anthracene as a colorless solid (0.90 g, 76%):
mp 243−246 °C; Rf 0.18 in hexane; 1H NMR (400 MHz, CDCl3) δ 2.44
(quint, J = 6.0 Hz, 4H, 6-H), 3.72 (t, J = 6.4 Hz, 4H, 7-H), 4.27 (t, J = 5.8
Hz, 4H, 5-H), 7.15 (d, J = 8.8 Hz, 4H, 3-H), 7.32−7.35 (m, 4H, 2-H),
7.39 (d, J = 8.8 Hz, 4H, 4-H), 7.73−7.76 (m, 4H, 1-H); 13C NMR (100
MHz, CDCl3) δ 29.8, 32.2, 65.1, 114.1, 124.6, 126.7, 129.9, 131.1, 132.1,
136.4, 157.8. Anal. Calcd for C32H28Br2O2: C, 63.59; H, 4.67. Found: C,
63.37; H, 4.62.
ASSOCIATED CONTENT
* Supporting Information
■
S
Additional UV−vis, PL, CV, and ECL figures, and NMR
reproduction for arbitrates and DAA1 and DAA2, atomic
coordinates and absolute energies of all structures are provided.
This material is available free of charge via the Internet at http://
pubs.acs.org.
Preparation of 9,10-Bis(4-bromopropoxy-2,6-
methylphenyl)anthracene. This compound was prepared from
9,10-bis(4-hydroxy-2,6-dimethylphenyl)anthracene by the same proce-
dure as described above (yield 79%): mp 219−224 °C; Rf 0.16 (n-
hexane); 1H NMR (400 MHz, CDCl3) δ 2.41 (s, 12H, 4-H), 2.45 (quint,
J = 6.0 Hz, 4H, 6-H), 3.80 (t, J = 6.4 Hz, 4H, 7-H), 4.10 (t, J = 6.4 Hz, 4H,
5-H), 7.11 (s, 4H, 3-H), 7.32−7.35 (m, 4H, 2-H), 7.73−7.75 (m, 4H, 1-
H); 13C NMR (100 MHz, CDCl3) δ 16.4, 30.4, 33.6, 69.2, 124.8, 127.1,
129.9, 130.7, 131.7, 134.5, 136.8, 154.8. Anal. Calcd for C36H36Br2O2: C,
65.47; H, 5.49. Found: C, 65.49; H, 5.48.
AUTHOR INFORMATION
Corresponding Author
*E-mail: natarajan@chemie.uni-siegen.de, schmittel@chemie.
uni-siegen.de.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
P.N. is grateful to all members in the group for their help as
needed, in particular, Dr. Qinghai Shu and Mr. Debabrata
Samanta for their assistance with ECL experiments and
computations, respectively. Also, P.N. would like to thank the
Alexander von Humboldt Foundation (AvH) for a postdoctoral
research fellowship. In addition, we are indebted to support of
this work by the DFG under Schm 647/18-1.
Preparation of DAA1.
A solution of 9,10-bis(4-
bromopropoxyphenyl)anthracene (0.90 g, 1.4 mmol) and N-methyl-
imidazole (0.51 g, 5.6 mmol) in 60 mL of acetone was refluxed under N2
for 10 h. After this period, a colorless precipitate formed and was isolated
by filtration. The solid was dissolved in DMF (4−5 mL), and NH4PF6
(0.50 g, 3.1 mmol) was added. The mixture was stirred for about 30 min
and poured in acetone (15 mL). The precipitate formed was isolated by
filtration and washed with small portions of cold water. The imidazolium
salt was purified by recrystallization from a water/acetone mixture to
yield 0.71 g (66%): mp 297−300 °C dec.; Rf 0.12 in methanol; IR (KBr,
cm−1) 3388, 3138, 3049, 2931, 2858, 1571, 1466, 1380, 1337, 1172,
1019, 890, 767, 655, 625; 1H NMR (400 MHz, CD3OD) δ 2.49 (quint, J
= 6.0 Hz, 4H, 6-H), 3.98 (s, 6H, 8-H), 4.24 (t, J = 5.6 Hz, 4H, 5-H), 4.56
(t, J = 7.2 Hz, 4H, 7-H), 7.17 (d, J = 8.4 Hz, 4H, 3-H), 7.30−7.34 (m, 8H,
2\4-H), 7.62−7.65 (m, 6H, 1\9-H), 7.77 (t, J = 5.6 Hz, 2H, 10-H), 9.1
(s, 2H, 11-H); 13C NMR (100 MHz, CD3OD) δ 30.9, 36.6, 48.5, 65.9,
115.6, 124.1, 125.1, 126.1, 127.9, 131.5, 132.9, 133.6, 137.9, 138.3,
159.5; ESI-MS m/z (%) 753.3 (100) [M − PF6]+. Anal. Calcd for
C40H40F12N4O2P2: C, 53.46; H, 4.49; N, 6.23. Found: C, 53.19; H, 4.61;
N, 6.27.
REFERENCES
■
(1) (a) Hercules, D. M. Science 1964, 145, 808. (b) Bard, A. J., Ed.
Electrogenerated Chemiluminescence; Marcel Dekker: New York, 2004.
(c) Richter, M. M. Chem. Rev. 2004, 104, 3003. (d) Faulkner, L. R.;
Tachikawa, H.; Bard, A. J. J. Am. Chem. Soc. 1972, 94, 691. (e) Bezman,
R.; Faulkner, L. R. J. Am. Chem. Soc. 1972, 94, 6317. (f) Keszthelyi, C. P.;
Tokel-Takvoryan, N. E.; Bard, A. J. Anal. Chem. 1975, 47, 249.
(g) Santhanam, K. S. V.; Bard, A. J. J. Am. Chem. Soc. 1965, 87, 139.
(h) Maness, K. M.; Bartelt, J. E.; Wightman, R. M. J. Phys. Chem. 1994,
98, 3993. (i) Wu, M.; Hercules, D. M. J. Phys. Chem. 1979, 83, 2203.
(j) Suk, J.; Wu, Z.; Wang, L.; Bard, A. J. J. Am. Chem. Soc. 2011, 133,
8676
dx.doi.org/10.1021/jo301616t | J. Org. Chem. 2012, 77, 8669−8677

The Journal of Organic Chemistry
Article
14675. (k) Suk, J.; Natarajan, P.; Moorthy, J. N.; Bard, A. J. J. Am. Chem.
Soc. 2012, 134, 3451. (l) Sartin, M. M.; Shu, C.; Bard, A. J. J. Am. Chem.
Soc. 2008, 130, 5354.
(2) (a) Sartin, M. M.; Zhang, H.; Zhang, J.; Zhang, P.; Tian, W.; Wang,
Y.; Bard, A. J. J. Phys. Chem. C 2007, 111, 16345. (b) Omer, K. M.;
Kanibolotsky, A. L.; Skabara, P. J.; Perepichka, I. F.; Bard, A. J. J. Phys.
Chem. B 2007, 111, 6612. (c) Vinyard, D. J.; Su, S.; Richter, M. M. J.
Phys. Chem. A 2008, 112, 8529.
(3) (a) Aubry, J.-M.; Pierlot, C.; Rigaudy, J.; Schmidt, R. Acc. Chem. Res.
2003, 36, 668. (b) Fudickar, W.; Linker, T. Chem.Eur. J. 2006, 12,
9276.
(4) Qi, H.; Peng, Y.; Gao, Q.; Zhang, C. Sensors 2009, 9, 674.
(5) Richards, T. C.; Bard, A. J. Anal. Chem. 1995, 67, 3140.
(6) (a) Ding, Z.; Quinn, B. M.; Haram, S. K.; Pell, L. E.; Korgel, B. A.;
Bard, A. J. Science 2002, 296, 1293. (b) Khalid, M. O.; Bard, A. J. J. Phys.
Chem. C 2009, 113, 11575.
(7) Guangmei, G.; Lin, Z.; Chen, X. Anal. Methods 2011, 3, 53.
(8) Few studies have been reported for diarylanthracenes to improve
the photo- or electrochemical stabilities in non-aqueous media; see:
(a) Sioda, R. E. J. Phys. Chem. 1968, 72, 2322. (b) Zehm, D.; Fudickar,
W.; Hans, M.; Schilde, U.; Kelling, A.; Linker, T. Chem.Eur. J. 2008,
14, 11429. (c) Matsubara, Y.; Kimura, A.; Yamaguchi, Y.; Yoshida, Z.
Org. Lett. 2008, 10, 5541.
(9) (a) Grampp, G.; Kapturkiewicz, A.; Salbeck, J. Chem. Phys. 1994,
187, 391. (b) Jou, J.-H.; Wang, C.-P.; Wu, M.-H.; Chiang, P.-H.; Lin, H.-
W.; Li, H.-C.; Liu, R.-S. Org. Electron. 2007, 8, 29. (c) Morley, J. O. J.
Chem. Soc., Perkin Trans. 2 1976, 1554. (d) Bard, A. J.; Santhanam, K. S.
V.; Maloy, J. T.; Phelps, J.; Wheeler, L. O. Discuss. Faraday Soc. 1968, 45,
167.
Chem. Acc. 2011, 130, 807. (c) Baciocchi, E.; Del Giacco, T.; Gerini, M.
F.; Lanzalunga, O. J. Phys. Chem. A 2006, 110, 9940.
(19) All commercially available ECL analytical instruments are based
on a co-reactant ECL technology. Furthermore, the limited potential
window of an aqueous solution using a Pt working electrode prompted
us to study only the oxidation of DAA1 and DAA2 in the presence of the
co-reactant.
(20) NMR measurement was performed after purification of the
electrolyzed solution of DAA1 or DAA2 as follows: The volatiles were
evaporated under vacuum, and supporting electrolyte was washed with
DCM. The resultant material was dissolved in methanol and filtered
through a short pad of Celite column.
(21) For example, see: (a) Fabbrizzi, L.; Licchelli, M.; Parodi, L.; Poggi,
A.; Taglietti, A. J. Fluoresc. 1998, 8, 263. (b) Malliaras, G.; Friend, R.
Phys. Today 2005, 58, 53.
(22) (a) Hammerich, O.; Parker, V. D. J. Am. Chem. Soc. 1974, 96,
4289. (b) Xiao, L.; Johnson, K. E. J. Electrochem. Soc. 2003, 150, E307.
(23) Zweig, A.; Maurer, A. H.; Roberts, B. G. J. Org. Chem. 1967, 32,
1322.
(24) Omer, K. M.; Ku, S.-Y.; Wong, K.-T.; Bard, A. J. Angew. Chem., Int.
Ed. 2009, 48, 9300.
(25) (a) Cruser, S. A.; Bard, A. J. J. Am. Chem. Soc. 1969, 91, 267.
(b) Fan, F. R. F.; Mau, A.; Bard, A. J. Chem. Phys. Lett. 1985, 116, 400.
(26) Chen, F.-C.; Ho, J.-H.; Chen, C.-Y.; Su, Y. O.; Ho, T.-I. J.
Electroanal. Chem. 2001, 499, 17.
(27) (a) McCord, P.; Bard, A. J. J. Electroanal. Chem. 1991, 318, 91.
(b) Nepomnyashchii, A. B.; Ono, R. J.; Lyons, D. M.; Bielawski, C. W.;
Sessler, J. L.; Bard, A. J. Chem. Sci. 2012, 3, 2628.
(28) (a) Li, J.; Yang, L.; Luo, S.; Chen, B.; Li, J.; Lin, H.; Cai, Q.; Yao, S.
Anal. Chem. 2010, 82, 7357. (b) Fan, F.-R. F.; Park, S.; Zhu, Y.; Ruoff, R.
S.; Bard, A. J. J. Am. Chem. Soc. 2009, 131, 937. (c) Suk, J.; Cheng, J.-Z.;
Wong, K.-T.; Bard, A. J. J. Phys. Chem. C 2011, 115, 14960. (d) Omer, K.
M.; Ku, S.-Y.; Cheng, J.-Z.; Chou, S.-H.; Wong, K.-T.; Bard, A. J. J. Am.
Chem. Soc. 2011, 133, 5492.
(10) (a) Schmittel, M.; Ganz, A. Chem. Commun. 1997, 999.
(b) Schmittel, M.; Luning, U.; Meder, M.; Ganz, A.; Michel, C.;
̈
Herderich, M. Heterocycl. Commun. 1997, 3, 493. (c) Schmittel, M.;
Michel, C.; Liu, S.-X.; Schildbach, D.; Fenske, D. Eur. J. Inorg. Chem.
2001, 1155. (d) De, S.; Mahata, K.; Schmittel, M. Chem. Soc. Rev. 2010,
39, 1555.
(29) Kang, M. S.; Kang, S. H.; Kim, S. G.; Choi, I. T.; Ryu, J. H.; Ju, M.
J.; Cho, D.; Lee, J. Y.; Kim, H. K. Chem. Commun. 2012, 48, 9349.
(30) Pyati, R.; Richter, M. M. Annu. Rep. Prog. Chem., Sect. C: Phys.
Chem. 2007, 103, 12.
(11) (a) Schmittel, M.; Lin, H.-W. Angew. Chem., Int. Ed. 2007, 46, 893.
(b) Lin, H.; Cinar, M. E.; Schmittel, M. Dalton Trans. 2010, 39, 5130.
(c) Schmittel, M.; Qinghai, S. Chem. Commun. 2012, 48, 2707.
(12) The electrophilic imidazolium group electronically separated
from the emitting unit should not affect the PL quantum yield and
radical ion stability; see: (a) Kohmoto, S.; Tsuyuki, R.; Hara, Y.; Kaji, A.;
Takahashi, M.; Kishikawa, K. Chem. Commun. 2011, 47, 9158. (b) Kim,
H. N.; Moon, J. H.; Kim, S. K.; Kwon, J. Y.; Jang, Y. J.; Lee, J. Y.; Yoon, J.
J. Org. Chem. 2011, 76, 3805. (c) Ahmed, N.; Shirinfar, B.; Youn, I. S.;
Bist, A.; Suresh, V.; Kim, K. S. Chem. Commun. 2012, 48, 2662.
(13) Boyd, T. J.; Schrock, R. R. Macromolecules 1999, 32, 6608.
(14) Kim, Y. H.; Kwon, S. K.; Yoo, D.; Rubner, M. F.; Wrighton, M. S.
Chem. Mater. 1997, 9, 2699.
(31) Kotha, S.; Ghosh, A. K.; Deodhar, K. D. Synthesis 2004, 4, 549.
(15) Hamal, S.; Hirayama, F. J. Phys. Chem. 1983, 87, 83.
(16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.;
Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith,
T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.;
Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J.
A. Gaussian 98, revision A.7; Gaussian, Inc.: Pittsburgh, PA, 1998.
(17) (a) Bock, H.; John, A.; Nather, C.; Havlas, Z.; Mihokova, E. Helv.
̈
Chim. Acta 1994, 77, 41. (b) Xia, Z.-Y.; Su, J.-H.; Wong, W.-Y.; Wang, L.;
Cheah, K.-W.; Tian, H.; Chen, C. H. J. Mater. Chem. 2010, 20, 8382.
(c) Adams, J. M.; Ramdas, S. Acta Crystallogr. 1979, B35, 679.
(18) (a) Wheeler, L. O.; Santhanam, K. S. V.; Bard, A. J. J. Phys. Chem.
1966, 70, 404. (b) Watanabe, S.; Shimodo, Y.; Morihashi, K. Theor.
8677
dx.doi.org/10.1021/jo301616t | J. Org. Chem. 2012, 77, 8669−8677